In a smart-grid or demand response setup, where an aggregator is directly controlling various distributed energy resources (DER) using power references, there will be a requirement for a power controller with estimation of flexibility for each of the distributed energy resources. The schematic of the information flow in smart-grid setup can be seen in FIG. 1. The task of the aggregator is to provide the electrical grid with some kind of regulation service which is traded through the market place.
For the aggregator to be able to provide regulation services it has to be able to control a portfolio of DERs, which can be both production and consumption units, the only requirement is that the DER has to be controllable from the aggregator. An example of a DER is a refrigeration system where the heat capacity of the stored goods is used to time shift power consumption and thereby assist the aggregator in providing regulation services to the electrical grid.
A method of U.S. Pat. No. 8,178,997 includes determining a desired power draw for a plurality of loads connected to an electrical grid, each of the plurality of loads connected to the electrical grid through a load supply control and being able to obtain a desired amount of energy from the grid in a desired time period, and transmitting a plurality of instructions through a communication network to a plurality of load supply controls to cause at least some of the loads in the plurality of loads to receive power from the electrical grid at different rates than other loads of the plurality of loads such that the desired power draw is obtained and such that each load of the plurality of loads receives its corresponding desired amount of energy in the desired time period. The plurality of loads can include certain implementations and may have one or more of the plurality of batteries or battery chargers, a plurality of hot water heaters, a plurality of refrigeration compressors, a plurality of dishwashers, a plurality of clothes dryers, a plurality of ice makers, or a plurality of swimming pool pumps. The plurality of batteries or battery chargers can include a plurality of batteries or battery chargers for plug-in vehicles. The method can further include determining, for each load, a rate of power draw required for the load to obtain its desired amount of energy in the desired time period. The desired amount of energy and the desired time period can be based upon a user input.
To provide regulation, i.e. response to short-term fluctuations, the aggregator or computer system of the aggregator can estimate the amount of power modulation achievable over the longest duration. In this case, the aggregator or by the aggregator or computer system of the aggregator from the controllable loads for any interval, called the assessment interval, during which some loads, called the active ensemble or managed ensemble of loads, are connected or can reasonably be expected to be connected. For this example, it is assumed that the aggregator or computer system of the aggregator can signal the loads to turn only on or off. However, the aggregator or computer system of the aggregator has the potential to continuously adjust the power drawn by each load rather than turning the loads on and off. This would allow the aggregator or computer system of the aggregator an additional degree of freedom in scheduling the power distribution on the loads.
The above prior art procedure of managing a portfolio of various kinds of DERs has turned out to be inadequate for the needs of a refrigeration system of a general kind.
More specifically, WO2010/069316 A1 relates to a method and a system to reduce losses of energy due to ripples, especially at the power grid, the ripples being short term power shortages or excess power. The method is based on the idea of shutting off energy consuming devices during a period of power shortage, if their operation is not necessary, and optionally to turn on such energy consuming devices during periods of excess power, if energy may be stored in them, especially when energy may be stored as some physical parameter or variable, being a part of the operation of the energy consuming devices, such as the temperature of a freezer.
Although the above mentioned method is adapted to a refrigeration system, nevertheless it can be improved as the above prior art method is mainly directed to reduce losses of energy due to ripples. It turns out that still, nevertheless, a refrigeration system demands for an improved kind of power consumption information to be communicated to an aggregator of a smart-grid setup. It turns out that there are some significant drawbacks when scheduling a refrigeration system with power consumption information which handles a maximum power consumption in the sense of nominal values and minimum power consumption values in the sense of zero values.